Hi Ralph,

I suppose he means this document:

http://www.blm.gov/nstc/library/pdf/TN428.pdfFor all you GCPs and accuracy freaks out there: keep in mind that in photogrammetry, the following rule is used: If you need a model with x cm accuracy, you need GCPs which have at least 1/3x accuracy. So, if you need to deliver a model with 6 cm accuracy, your GCPs should measured with at least 2 cm accuracy.

Additionally, the ground sampling distance (GSD) of the camera has to be taken into account as well as its real resolving power. A GSD of 10 cm does not mean that the true resolving power is 10 cm: this depends on many variables, the MTF of the optical system and the contrast of the subject being the prime variables.

Also, keep in mind that the georeferencing errors PS delivers do NOT say anything about the accuracy of your model, they just inform your on its global georeferencing accuracy. Obviously, a good georeferening is more likely to be more accurate. A true accuracy assessment can only be obtained with an independent set of ground control measurements against which your model can be validated.

Cheers,

Geert